the best place is there where the difference in pressure between the inside and outside is greatest, you can measure this with a pressure gauge, but generally this is halfway the hood at the sides. you want to make sure you stay ahead of the high pressure bubble in front of the windshield or you"ll have air going in the vents.
generally the air on the underside of a vehicle is moveing faster especially if you have an undertray so extraction there would be better, but there's a few drawbacks. if the underside of the car is less than optimal you'll likely have a lot of turbulence there, wich you don't want to add to that, and wich might prevent the extractors form working propperly. air that comes out at the top will move over the smooth car body.
also hot air, wich will be found in the engine bay, will rise... venting at the bottom will expell the coolest air but leave the hot air trapped at the top, hood vents will help here.
one car that comes to mind as a good examle of hood vents is the opel kadett E gsi (i owned the less sporty version without the vents)
here nicely highlighted in yellow
http://upload.wikimedia.org/wikipedi...tt_GSi_16V.jpg